74 Chapter 4 The Bland-Altman analysis showed an overall mean difference of -1% between FVIII activity and plasma concentration, with an SD of 64% (limits of agreement are -127% to 125%) and more variability in the lower measurement range. The relative differences were normally distributed. When cut off at a mean of 40% (the upper activity limit for haemophilia A diagnosis), the mean difference was -13% in the lower range (mean <40%) and a mean difference of 20% was found in the upper range (mean >40%). In Figure 2, Bland-Altman plots are demonstrated for four different subgroups, all with the overall mean and limits of agreement. Figure 2. Bland-Altman plots of relative difference compared to mean (FVIII plasma concentration – FVIII activity)/2). Subgroups are illustrated per panel and with different symbols, see subtitles. Overall mean of -1%, limits of agreement are -127% to 125%. Linear regression analyses demonstrated the correlation between determinants and the relative differences of the results from LC-MS/MS and OSA. Relevant results are demonstrated in Table 2 and two significant results are illustrated in the boxplots of Figure 3. The relative differences of samples with anti-FVIII antibodies (≥0.6 BU/mL, n = 5) were 133% (95% CI: 81; 185%) higher compared to the relative differences of samples without anti-FVIII antibodies. The relative differences of samples with exogenous FVIII products (n = 58), the ‘exposed’ group, were -37% (95% CI: -65; -9%) when compared to samples without exogenous FVIII (n = 29), the ‘unexposed’ group. When multivariable linear regression was performed, the found correlation between differences and

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